When a book is opened for reading, the book is damaged in some cases. In particular, an old book may be damaged or destroyed when being opened. For example, an ancient rolled document that was burnt by the eruption in ancient Roman times was discovered in Italy. The ancient document is difficult to be interpreted with unaided eyes because it is entirely blackish, and is difficult to be unrolled because it is fragile.
NPL 1 describes that Greek letters written in the rolled ancient document that was burnt by the eruption were extracted from the ancient document. It is reported in NPL 1 that three-dimensional data of the ancient document was generated by X-ray phase-contrast tomography and pixel patterns considered to be isolated Greek letters were found in the three-dimensional data.
NPL 2 describes a technique of visualizing three-dimensional data. In the technique of NPL 2, an isosurface having a specific data value is automatically specified from three-dimensional data having distribution of continuous data values.